Unfortunately, there are some unsanitary conditions and health warnings, even in the city. Tap water is largely undrinkable for a variety of reasons. Old legislation and low capital investment in water systems and treatment left most of the water un-potable (un-drinkable). Most hotels provide bottled water and have potable water delivered through their taps using an on-site purification system; there will be a note in your room to advise you if this is true. All street vendors selling refreshments will sell you purified bottled water, but make sure that the cap is sealed. All commercially produced beverages, including bottled and tinned water, fizzy drinks, wine, beer, spirits, etc will be perfectly safe for you to drink. To make tap water safe, boil it for at least a few minutes; longer in locations situated at higher altitudes as the water boils at lower temperatures there. Water purification tablets and drops are available, but these generally have an adverse affect on the water's taste.
There are many excellent and English speaking doctors in Mexico. Some hospitals even accept American insurance and health plans. If in need, visitors can check with the hotel as larger hotels often keep doctors on premises or on call. If not, hotels can usually refer visitors to a doctor.